I am not 100% sure, but this plug, I think A 000 987 03 45 (?), may be available at MB (it is here in Europe, if it is this part - you do not see it in the picture in the EPC, but it is there together with the clamp on the parts list, it may be this one you ask about). It is $ 3-4 here.
Keep the original small clamp, I think it is quite rare.
I am not sure if the fluid reservoir you have is available in its exact original shape. I would keep it (I think this is what you are planning to do). It has been discussed in another post that a good wash with soap water or washing powder may significantly improve its look (yours does not look that bad, actually).
The sealing ring between the master and booster is available in MB, ca $5 (A 004 997 32 45).
I do not know if master cylinder kits include the reservoir grommets.
What I see is that the grommet appears as a separate spare part "As of ident. no.: 043 002980" and it has a part number A 000 431 09 35. This chassis number marks also the change of master cylinder types or PN. Could be the earlier one had grommets built in the later one - separate - but I am not sure, you would need to confirm.
There are also some repair kits in MB, but do not know if applicable in your case.